1. Field of the Invention
The invention pertains to image analysis, more specifically to a mechanism of processing interlaced video signal whereby motion between successive fields of the same or opposite parity are detected.
2. Description of Related Art
The NTSC and PAL video standards are in widespread use throughout the world today. Both of these standards make use of interlacing video signals in order to maximize the vertical refresh rate thereby reducing wide area flicker, while minimizing the bandwidth required for transmission. With an interlaced video format, half of the lines that make up a picture are displayed during one vertical period (i.e. the even field), while the other half are displayed during the next vertical period (i.e. the odd field) and are positioned halfway between the lines displayed during the first period. While this technique has the benefits described above, the use of interlacing can also lead to the appearance of artifacts such as line flicker and visible line structure.
It is well known in the prior art that the appearance of an interlaced image can be improved by converting it to non-interlaced (progressive) format and displaying it as such. Moreover, many newer display technologies, for example Liquid Crystal Displays (LCDs) are non-interlaced by nature, therefore conversion is necessary before an image can be displayed at all.
Numerous methods have been proposed for converting an interlaced video signal to progressive format. For example, linear methods have been used, where pixels in the progressive output image are generated as a linear combination of spatially and/or temporally neighboring pixels from the interlaced input sequence. Although this approach may produce acceptable results under certain conditions, the performance generally represents a trade off between vertical spatial resolution and motion artifacts. Instead of accepting a compromise, it is possible to optimize performance by employing a method that is capable of adapting to the type of source material. For instance, it is well known that conversion from interlaced to progressive format can be accomplished with high quality for sources that originate from motion picture film or from computer graphics (CG). Such sources are inherently progressive in nature, but are transmitted in interlaced format in accordance with existing video standards. For example, motion picture film created at 24 frames per second using a process known as 3:2 pull down, where 3 fields are derived from one frame and 2 are derived from the next, so as to provide the correct conversion ratio. Similarly, a computer graphics sequence created at 30 frames per second is converted to interlaced video at 60 fields per second using a pull down ration of 2:2, where 2 fields are derived from each CG frame. By recognizing that a video sequence originates from a progressive source, it is possible for a format converter to reconstruct the sequence in progressive format exactly as it was before its conversion to interlaced format.
Unfortunately, video transmission formats do not include explicit information about the type of source material being carried, such as whether the material was derived from a progressive source. Thus, in order for a video processing device to exploit the progressive nature of film or CG sources, it is first necessary to determine whether the material originates from a progressive source. If it is determined that the material originates from such a source, it is furthermore necessary to determine precisely which video fields originate from which source frames. Such determination can be made by measuring the motion between successive fields of an input video sequence.
It is common to measure at least two different modes of motion in determining the presence of a film source. Firstly, it is common to measure the motion between a given video field and that which preceded it by two fields. In this case, motion can be measured as the absolute difference between two pixels at the same spatial position in the two fields. A measure of the total difference between the two fields can be generated by summing the absolute differences at the pixel level over the entire field. The quality of the motion signal developed in this way is fairly high, since the two fields being compared have the same parity (both odd or both even) and therefore corresponding samples from each field have the same position within the image. Thus, any difference that is measured between two pixels will largely be the result of motion. The measure of motion between the first and third fields of the three fields that are derived from the same motion picture frame will be substantially lower than the measurements obtained during the other four fields, since the two fields being compared are essentially the same and differ only in their noise content. This does not provide sufficient information to avoid artifacts under certain conditions when a film sequence is interrupted. Also, in the case of an input sequence derived from film or CG in accordance with a 2:2 pull down ratio, no useful information is provided whatsoever.
A second mode of motion that can be measured is the motion between successive fields which are of opposite parity (one odd and the other even). Although this mode of measurement overcomes the limitations of the above, it is inherently a more difficult measurement to make since a spatial offset exists between fields that are of opposite parity. This is particularly true in the presence of noise and/or limited motion. A number of methods have been proposed in the prior art for the measurement of motion between fields of opposite parity. One such method is disclosed in U.S. Pat. No. 6,647,062 B2 entitled “Method and Apparatus for Detecting Motion and Absence of Motion between Odd and Even Video Fields”, the contents of which are incorporated herein by reference.
The method described in U.S. Pat. No. 6,647,062 describes a motion detection method in which for either odd or even current field, the selected pixels for detecting motion between the current and the previous fields are not vertically symmetrical with respect to the missing pixels to be interpolated, depending on the parity of the current field. Moreover, in the system described in U.S. Pat. No. 6,647,062 and others in the prior art, the footprint of the selected pixels for detecting motion near a specified position in the temporal-vertical plane is not temporally symmetrical with respect to the missing pixel to be interpolated.